The present system relates generally to cardiac rhythm management systems and particularly, but not by way of limitation, to such a system providing a diagnostic based at least in part on the variability of intervals between atrial heart contractions.
When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body""s circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a xe2x80x9cleadxe2x80x9d) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as xe2x80x9ccapturingxe2x80x9d the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly. Such pacers coordinate atrial and ventricular contractions to improve pumping efficiency.
Cardiac rhythm management systems also include defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Such defibrillators also include cardioverters, which synchronize the delivery of such stimuli to portions of sensed intrinsic heart activity signals. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn""t allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock, also referred to simply as a xe2x80x9cshock.xe2x80x9d The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. Other cardiac rhythm management systems combine the functions of pacers and defibrillators.
One problem faced by physicians treating cardiovascular patients is assessing patient well-being, either for providing a prognosis or for adjusting a therapy to improve the patient""s prognosis. Ventricular heart rate variability (xe2x80x9cHRVxe2x80x9d) is thought to provide one such assessment of cardiovascular health. The time interval between intrinsic ventricular heart contractions changes in response to the body""s metabolic need for a change in heart rate and the amount of blood pumped through the circulatory system. For example, during a period of exercise or other activity, a person""s intrinsic ventricular heart rate will generally increase over a time period of several or many heartbeats. However, even on a beat-to-beat basis, that is, from one heart beat to the next, and without exercise, the time interval between intrinsic heart ventricular contractions varies in a normal person. These beat-to-beat variations in intrinsic ventricular heart rate are the result of proper regulation by the autonomic nervous system of blood pressure and cardiac output; the absence of such variations indicates a possible deficiency in the regulation being provided by the autonomic nervous system.
The autonomic nervous system itself has two components: sympathetic and parasympathetic (or vagal). The sympathetic component of the autonomic nervous system is relatively slow acting, and is associated with a tendency to raise heart rate, blood pressure, and/or cardiac output. The parasympathetic/vagal component of the autonomic nervous system, which provides a relatively faster response than the sympathetic component, is associated with a tendency to reduce heart rate, blood pressure, and/or cardiac output. A proper balance between the sympathetic and parasympathetic components of the autonomic nervous system is important. Therefore, an indication of this balance of the components of the autonomic nervous system, which is sometimes referred to as xe2x80x9cautonomic balance,xe2x80x9d xe2x80x9csympathetic tone,xe2x80x9d or xe2x80x9csympathovagal balance,xe2x80x9d provides a useful indication of the patient""s well-being.
One technique for providing an indication of the balance of the components of the autonomic nervous system is provided by the beat-to-beat heart rate variability, as discussed above. More particularly, intrinsic ventricular contractions are detected. The time intervals between these contractions, referred to as the R-R intervals, are recorded after filtering out any ectopic contractions, that is, ventricular contractions that are not the result of a normal sinus rhythm. This signal of R-R intervals is typically transformed into the frequency-domain, such as by using fast Fourier transform (xe2x80x9cFFTxe2x80x9d) techniques, so that its spectral frequency components can be analyzed. Two frequency bands are of particular interest: a low frequency (LF) band in the frequency (xe2x80x9cfxe2x80x9d) range 0.04 Hzxe2x89xa6fxe2x89xa60.15 Hz, and a high frequency (HF) band in the frequency range 0.15 Hzxe2x89xa6fxe2x89xa60.40 Hz. The HF band of the R-R interval signal is influenced only by the parasympathetic/vagal component of the autonomic nervous system. The LF band of the R-R interval signal is influenced by both the sympathetic and parasympathetic components of the autonomic nervous system. Consequently, the ratio LF/HF is regarded as a good indication of the autonomic balance between sympathetic and parasympathetic/vagal components of the autonomic nervous system. An increase in the LF/HF ratio indicates an increased predominance of the sympathetic component, and a decrease in the LF/HF ratio indicates an increased predominance of the parasympathetic component. For a particular heart rate, the LF/HF ratio is regarded as an indication of patient wellness, with a lower LF/HF ratio indicating a more positive state of cardiovascular health.
The present inventors have recognized that such diagnostic techniques based on ventricular heart rate variability have certain limitations. For example, cardiac rhythm management systems typically include operational modes that do not track atrial heart rate. In such systems, ventricular heart rate variability is not necessarily representative of sinus rhythm and, therefore, is not necessarily representative of the balance between sympathetic and parasympathetic components of the autonomic nervous system. Moreover, even when the cardiac rhythm management system is operating in a mode that tracks atrial heart rate, R-R intervals that are associated with premature ventricular contractions (PVCs) must typically be ignored in any determination of sympathetic/parasympathetic balance based on ventricular heart rate variability. This reduces the amount of available data upon which the determination of sympathetic/parasympathetic balance is based. Furthermore, cardiac rhythm management systems may also include ventricular rate smoothing or ventricular rate stabilization algorithms. Because such techniques intentionally reduce or eliminate ventricular heart rate variability, they further confound any determination of sympathetic/parasympathetic balance based on ventricular heart rate variability. For these and other reasons, the present inventors have recognized that there is a need to provide improved diagnostic information indicative of sympathetic/parasympathetic balance.
This document describes a cardiac rhythm management system that uses atrial heart rate variability to provide a diagnostic indication of patient well-being that reflects an autonomic balance between the sympathetic and vagal components of the autonomic nervous system. Such diagnostic information is available even when the cardiac rhythm management devices provides ventricular rate control therapy that does not track the atrial heart rate. Moreover, such atrial heart rate variability diagnostic information provides a more direct indication of sinoatrial rate, without being confounded by the presence of premature ventricular contractions (PVCs) or requiring techniques for minimizing the effects of PVCs. Furthermore, the atrial heart rate variability diagnostic information is available even when ventricular rate smoothing or stabilization algorithms are being used. The system, which includes both methods and apparatuses, provides graphical and other display techniques for presenting the atrial heart rate variability diagnostic information. In one embodiment, the system provides time-domain processing of atrial heart rate variability information to provide an indication of patient well-being based on the frequency content of an atrial heart rate interval signal. Other aspects of the invention will be apparent on reading the following detailed description, drawings, and claims.